1. Field
One embodiment of the invention relates to an arrangement method of a servo pattern of a patterned medium, a patterned medium, and an information storage device.
2. Description of the Related Art
Heretofore, a magnetic disk medium of a hard disk or the like is constituted of a continuous film medium formed of granular magnetic grains with a grain size of 10 nm or less, and one bit is constituted of several tens of magnetic grains. Concomitant with the increase in the recording density of a recording medium in recent years, further micronization of magnetic grains is advanced. However, when the volume of an individual magnetic grain is reduced by the micronization, the stability of the magnetization state is reduced, and hence there arises a problem that the recording medium is deteriorated by heat fluctuation.
Thus, attention is paid not to granular magnetic grains, but to a patterned medium in which one bit is constituted of one dot of a record single-domain film. In a vertical magnetic storage medium used as the patterned medium, isolated magnetic dots are formed on the disk medium in the track direction, and an information unit (bit) is recorded by the magnetic dot, and the medium is called a bit-patterned medium (BPM). In the BPM, there are a data area configured to record data, and servo pattern area configured to record servo information.
In a magnetic disk device in which such a pattern-processed BPM is used, the direction of magnetization of a dot (data bit) for recording data changes in accordance with the value of data in the data area, whereas the servo information is embedded in advance in an area (servo pattern area) in which single magnetic domains are formed. The technique of the patterned medium for embedding and forming in advance the servo information as a pattern including unevenness of a magnetic layer is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2006-147112. In this case, the magnetization state of the servo pattern area for storing servo information is normally one of upward direction in which magnetization is directed to the surface side of the medium, and oppositely downward direction.
In such a servo pattern area, there are an address part for storing cylinder information and sector information, and position part. The area (although indicative of a volume, hereinafter simply referred to as an area) of a dot of the position part is close to the area of a dot in the data area. On the other hand, regarding the area of a dot in the address part, if the data length is made large in order to secure reliability, although the pattern is divided in the radial direction in the lower-order bits, the pattern is not divided in the radial direction in the higher-order bits. This is because when the data length is large, in the higher-order bits, there is a series of “1”, hence the number of dots each expressing “1” becomes large, and the dots form a group to make the area larger, whereas in the lower-order bits, “1” and “0” frequently change, thus the dots are dispersed, and hence the area does not become large.
In the servo pattern area, all the bit magnetization states are in the same direction, and hence magnetic fluxes generated from adjacent bits strike against each other. Further, when the dot area is small, even a single magnetic domain is magnetically stable by virtue of the high coercive force (Hc). When the dot area is large, in the record single-domain film, the coercive force (Hc) lowers to make the state magnetically unstable, and a phenomenon in which part of the magnetization is inverted is caused. When once the magnetization of the bits in the servo pattern area is inverted, this inversion is propagated, thereby destroying servo information in a plurality of tracks.
In servo signals recorded as the dots in the servo pattern area, all the bit magnetization states are set in the same direction in the recording, and the magnetization once recorded is not recorded again. As a result of this, if a phenomenon in which part of the magnetization is inverted is caused at a part of the servo pattern area in which the dot area is large, there has been a problem that the track positioning accuracy of the magnetic head is deteriorated.